Plasma Sheet Variations Observed on Kinetic Timescales

¹Geophysics Program, University of Washington, Seattle
²Space Sciences Laboratory, University of California, Berkeley
³Laboratory for Extraterrestrial Physics, NASA/GSFC, Greenbelt, MD

We examine high-time resolution plasma and magnetic field data from WIND perigee passes through the near-Earth plasma sheet during auroral events associated with high earthward ion velocity moments. Full 3-D ion and electron distributions are obtained every 3 seconds, and the magnetic field is sampled every 46 milliseconds. Variations in the plasma distributions and moments occur on timescales on the order of the local proton gyroperiod. The ion velocity moment typically changes by over 100 km/s from one 3-second distribution to the next. Averaging over several distributions smears out these variations and yields lower velocity moments. The magnetic field changes in direction and magnitude on timescales faster than the proton gyroperiod with dB/dt reaching values in excess of 30 nT/s. Magnetic field power spectra indicate that there can be significant wave power at frequencies on the order of the local proton gyrofrequency consistent with particle results. These results highlight the importance of high-time resolution plasma measurements to properly describe dynamic velocity moment events in the plasma sheet associated with aurora. The timescales of the observed fluctuations are much faster than convection timescales that are usually considered when discussing plasma sheet dynamics. Our study shows that kinetic processes are determining the plasma sheet dynamics.

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